1. Field of the Invention
The present invention relates to wireless communication systems used for car telephones and portable telephones, etc. and their methods.
2. Related Art
Wireless communication systems such as car telephones and portable telephones are based on a multiple access system in which communications are performed simultaneously between one base station and multiple users (mobile stations). Recently, the CDMA (Code Division Multiple Access) system, which allows efficient use of frequencies, is used as this multiple access system.
The transmission block of a conventional CDMA system base station (hereafter simply referred to as "base station") is described below using a block diagram in FIG. 1. As shown in FIG. 1, a conventional base station has spread modulation section 101 which spreads the transmit data to be transmitted to each user using a spread code assigned to each user, quadrature modulator 102 which quadrature-modulates the spreading transmit data, and gain control amplifier 103 which adjusts the transmission power of the quadrature-modulated transmit signals which is modulated according to a power control signal. The conventional base station also has adder circuit 104 which adds each transmit signals whose transmission power has been amplified, mixer 105 which multiplies the added transmit signals by a local frequency and modulates it into a radio frequency band, amplifier 106 which amplifies the transmission power of the transmit signals modulated into the radio frequency band by a constant amplification factor, and antenna 107 which transmits the transmit signals.
The flow of the transmit data/signals at the conventional base station is described below: Transmit data A directed to user A is spreading using a spread code assigned to user A by spread modulation section 101a and quadrature-modulated by quadrature modulator 102a. The transmission power of transmit signals A is adjusted by gain control amplifier 103a based on a power control signal. The transmit signals of other users is each spreading, quadrature-modulated and its transmission power adjusted in the like manner. Each user's transmit signals whose transmission power has been adjusted is added by adder circuit 104, modulated into a radio frequency band by mixer 105, with the transmission power amplified by amplifier 106 by a constant amplification factor, and transmitted by radio through antenna 107.
Each user receives the signals transmitted from the base station and after converting the frequency of the receive signals, inversely spreads it using an assigned spread code to extract the data transmitted from the base station directed to the mobile station. In this inverse spreading process, the transmit signals of other channels directed to other stations acts as noise. Each user indicates the base station the transmission power of the next transmit data directed to the station based on an S/I ratio which is the ratio of the transmission power of the receive data directed to the station to the transmission power of interference (noise).
At this point, when some users perform burst transmissions capable of transmitting a large volume of data, the conventional base station has the problem of transmit errors caused by an instantaneous variation of the S/I ratio, resulting in a deterioration of the quality of communications by the other users.